Fixes NPE when a Response does not provide headers (#855)

Fixes #853

There are some scenarios reported where a server does
not provide headers with the response.  While this is
not typically expected, it's simple enough for Feign to
be resilient to it.  This change checks the headers provided
in the builder and if none are provided, an empty map is
used in it's place.

Author: kdavisk6

core/src/main/java/feign/Response.java

@@ -22,6 +22,7 @@
 import java.nio.charset.Charset;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.LinkedHashMap;
 import java.util.LinkedList;
 import java.util.Locale;
 import java.util.Map;
@@ -49,7 +50,9 @@ private Response(Builder builder) {
     this.status = builder.status;
     this.request = builder.request;
     this.reason = builder.reason; // nullable
-    this.headers = Collections.unmodifiableMap(caseInsensitiveCopyOf(builder.headers));
+    this.headers = (builder.headers != null)
+        ? Collections.unmodifiableMap(caseInsensitiveCopyOf(builder.headers))
+        : new LinkedHashMap<>();
     this.body = builder.body; // nullable
   }
 

core/src/test/java/feign/ResponseTest.java

@@ -71,4 +71,14 @@ public void headerValuesWithSameNameOnlyVaryingInCaseAreMerged() {
         Arrays.asList("Cookie-A=Value", "Cookie-B=Value", "Cookie-C=Value");
     assertThat(response.headers()).containsOnly(entry(("set-cookie"), expectedHeaderValue));
   }
+
+  @Test
+  public void headersAreOptional() {
+    Response response = Response.builder()
+        .status(200)
+        .request(Request.create(HttpMethod.GET, "/api", Collections.emptyMap(), null, Util.UTF_8))
+        .body(new byte[0])
+        .build();
+    assertThat(response.headers()).isNotNull().isEmpty();
+  }
 }